1. Technical Field
Example embodiments of the present invention relate in general to a wireless communication apparatus and more specifically to a transmission apparatus.
2. Related Art
Next generation wireless communication systems such as Long Term Evolution (LTE) of 3rd Generation Partnership Project (3GPP) and Worldwide Interoperability for Microwave Access (WiMAX) of Institute of Electrical and Electronics Engineers (IEEE) use wideband signals that are modulated through Orthogonal Frequency Division Multiplexing (OFDM) for high spectral efficiency.
However, systems using OFDM such as LTE and WiMAX reduce power efficiency because of high peak-to-average power ratio (PAPR) when transmitting signals using a general linear amplifier.
To overcome this limitation, much research has been conducted to maximize power efficiency of a transmission apparatus by using a power amplifier such as a Doherty amplifier or envelope tracking amplifier.
However, the Doherty amplifier or envelope tracking amplifier has a characteristic in that a non-constant envelope signal is applied to an input of the amplifier, and thus disadvantageously the nonlinearity of the amplifier increases when a signal with high PAPR is applied to an input of the amplifier. This means that there are limitations in enhancing efficiency of a transmission apparatus that uses at least one of the above-described amplifiers.
Accordingly, in order to overcome the above-described limitations, a transmission apparatus that uses a switching power amplifier has been proposed.
A switching power amplifier has an input signal limited as an envelope signal with a constant magnitude, and thus a transmission apparatus that uses the switching power amplifier generates an input signal using an envelope delta-sigma modulator (EDSM) or envelope pulse-width modulator (EPWM).
The switching power amplifier always operates in a saturation region by using the above-described input signal encoder, thereby securing linearity of and obtaining high switching efficiency of even a non-constant envelope signal.
As such, a transmission apparatus that uses a modulated input signal is referred to as a class-S transmission apparatus. In addition, a transmission apparatus that applies a multi-level modulated input signal to the switching power amplifier is referred to as an advanced class-S transmission apparatus.
The signal quality of the class-S transmission apparatus is mainly determined by a modulator. In particular, a difference in the signal quality occurs due to a dimension and an output level of the modulator, which indicate structural performance of the modulator. Accordingly, in order to increase the signal quality of the class-S transmission apparatus, optimization of the dimension and the output level of the modulator are required.
However, in order for the class-S transmission apparatus to operate at an optimum performance, matching between a modulator and a unit that is positioned at a front or rear end thereof is also very important in addition to the structural performance of the modulator.
That is, the signal quality of the class-S transmission apparatus largely depends on how an input/output signal of the modulator is processed to match between the modulator and units that are positioned at front and rear ends thereof.
Accordingly, further research on an apparatus and method for processing an input/output signal of the modulator is needed.